1. Field of the Invention
Laser devices in which the active laser member is in the form of a slab. 2. Description of the Prior Art
Increased demands are being placed on laser sources, both as to military and industrial applications. Use of the slab configuration for the active laser member in place of the cylindrical rod configuration, offers potential for increasing the average output power limit of such member, for any given material of which such member is composed. Furthermore, the ability to vary the average power output over wide ranges without noticeably affecting beam divergence or efficiency is an intrinsic and valuable property of slab lasers.
While a slab laser is simple in concept, its use can be complicated by practical considerations. FIGS. 1a and 1b illustrate the pertinent geometical features of a prior art laser slab of rectangular cross section. Laser radiation can enter and exit via end faces A and propagate along path P down the length of the slab by successive total internal reflections from optically polished top and bottom faces B. In operation, coolant (not shown) is passed over such faces B while side faces C are thermally insulated. Pump radiation enters the slab via the faces B. Faces A must remain unobstructed; hence, a seal D is utilized to contain the coolant flow.
Efficiency of the slab laser depends primarily on the total internal volume of the active laser medium which is swept out by the laser beam as it propagates down the length of the slab. This swept volume, in turn, is influenced by the coupling technique used to introduce and extract laser radiation from the slab and the width of the coolant seal necessary for reliable operation. In general, it has been the practice to taper the ends of the slab for Brewster angle coupling as in FIG. 1a. This gives an input and output beam path that is parallel to the longitudinal axis of symmetry of the slab and permits an in-line resonator design. It also affords flexibility in selection of the optical pump configuration, and it permits linearly polarized laser output even at high average power. Such FIG. 1a coupling configuration, however, tends to: (1) limit the volume of the slab swept by the beam, often to less than fifty percent of the total slab volume, due to seal width requirements; (2) complicate construction due to the fragility and configuration of the Brewster angle ends of a laser slab of material such as Nd:YAG (3) introduce thermal imbalances in the Brewster angle ends which can degrade laser performance due to optical distortion; and, (4) introduce asymmetry in pump enclosure geometry, due to off-set of the Brewster angle ends, with consequent reduction in pumping efficiency and added complexity of enclosure design.